Real‐time Observation of Structural Dynamics Triggering Excimer Formation in a Perylene Bisimide Folda‐dimer by Ultrafast Time‐Domain Raman Spectroscopy

Abstract In π‐conjugated organic photovoltaic materials, an excimer state has been generally regarded as a trap state which hinders efficient excitation energy transport. But despite wide investigations of the excimer for overcoming the undesirable energy loss, the understanding of the relationship between the structure of the excimer in stacked organic compounds and its properties remains elusive. Here, we present the landscape of structural dynamics from the excimer formation to its relaxation in a co‐facially stacked archetypical perylene bisimide folda‐dimer using ultrafast time‐domain Raman spectroscopy. We directly captured vibrational snapshots illustrating the ultrafast structural evolution triggering the excimer formation along the interchromophore coordinate on the complex excited‐state potential surfaces and following evolution into a relaxed excimer state. Not only does this work showcase the ultrafast structural dynamics necessary for the excimer formation and control of excimer characteristics but also provides important criteria for designing the π‐conjugated organic molecules.

. References-19 by using a 3 mm sapphire plate. After amplification, Vis-NOPA is compressed via chirped mirrors, followed by fine-tuning with a wedge prism pair. The temporal width reaches near 7 fs characterized by SHG-FROG. The probe pulse is generated with a 2 mm YAG plate, allowing us to detect the 600-900 nm region. The time delay between pump and probe is controlled by the motorized linear stage (M-VP-25XA (Newport)). Polarizations of two pulses are parallel at the sample position to maximize vibrational coherence signals. The signals are obtained by a CCD detector (Stressing, FL-3030). As in the TR-ISRS experiment, in order to minimize GDD, we use an ultrathin wall aperture (500 μm UVFS) flow cell (48/UTWA2/Q/0.2, Starna) with 200 μm optical path length. Also, a microannular gear pump (mzr-4622 M2.1) with Teflon tubing is used for flowing the sample solutions which can remove the thermal-lens effect due to the 10 kHz repetition rate of the laser and avoid photo-degradation of the sample. We usually prepare 2-3 ml sample solutions for the experiments (c0 = 5.0 x 10 -4 M). The pump pulse is modulated at 5 kHz by a mechanical chopper (MC1F60, Thorlabs), which allows data processing in a shot-to-shot fashion. 30000 x 2 pulses are averaged for each time delay.

Discussion on analysis of the excited-state Raman
Here, we would like to emphasize a few findings to rationalize our analysis. 1) Analysis of the transient absorption: Although the isosbestic point is around 800 nm, the broadband TA spectra indicated that the ESA band over the NIR NOPA region holds its spectral feature with time ( Figure S15). This suggests that the electronic structures in the Frenkel-like state could be quite similar to those in the excimer state. Furthermore, the initial TA spectrum in a polar solvent is different from those in a weak polar solvent (Figur e 2 and S4-S5), indicating that the initial Frenkel-like state is also mixed with the CT state like the excimer state. 2) Temperature-dependent experiments: The temperature-dependent experiments strongly support that the excimer formation process is accompanied by structural changes. It is noted that time-resolved fluorescence results demonstrated the changes in the emission spectra with the excimer formation process (i.e., the inversion of A0-0/A0-1) in the previous report 4 . While the Frenkel-like emission was observed below the melting point, the emission spectrum dramatically changed into an excimer-like feature above the melting point. The striking change in the emission spectrum above the melting point is a strong evidence for the structural dynamics in the excimer formation. 3) Excimer formation observed in various PBI dimers: We recall the excimer formation process in numerous PBI dimers that show different coupling strengths as described in the main text. The excimer formation process in PBI dimers occurs with the ultrafast timescale of 200 fs irrespective of their coupling strength. Furthermore, as we mentioned before, negligible changes in TA spectra were observed during the excimer formation process. Overall, these results suggest that the excimer formation process is adiabatic rather than dia batic. 4) Gradual shifts of xOOP mode: If the transition is close to the diabatic process, the isosbestic point is expected. However, Raman dispersions of xOOP mode show gradual blue-shifts without the isosbestic point. Especially, the short-time Fourier transform (STFT) using broadband TA also shows a gradual blue-shift in Raman frequency around 110 cm -1 ( Figure S13). The broad visible NOPA (< 10 fs) triggers the excited state wavepackets near the FC geometry. Furthermore, if xOOP mode is an exclusive feature of the excimer state, the distinct rise (OOP mode for excimer state) and decay (initial OOP mode for Frenkel-like state) should be observed. However, STFT only shows gradual blue-shifts in Raman frequency of xOOP mode. The STFT result reveals that the xOOP mode is not the exclusive signature of the excimer state itself. Taken all together, the excimer formation process is not only a change of the configuration percentage in admixture of locally excited (LE) and charge transfer (CT) wavefunctions but also accompanied by the structural change.
Also, we present the averaging procedure to improve the quality of experimental results such as the frequency and power of the excited state Raman spectra. As we described in the experimental details, we used zero-padding and Hanning window function to improve the quality of FT power spectra. To minimize the artifact induced by the long-term effects such as the fluctuation of the excitation power and/or laser stability, the set of all time delays (ΔT) was measured repeatedly; (set 1 [-3.3 → 0 → ··· → 100 ps] → set 2 [-3.3 → 0 → ··· → 100 ps] → ···). Especially, Figure S16 indicate that FT power spectra are converged after 15 or more of raw TR-ISRS decay averages. Furthermore, other noises were significantly suppressed by the averaging effect. As shown in Figure S16, the comparison of the excited-state Raman spectra at 0.33 and 1 ps depending on the averaged decays confirm the following points: 1) the frequency of xOOP mode is blue-shifted and 2) the amplitude of xOOP mode increases.

Additional discussion on temperature-dependent TA and time-resolved fluorescence measurements
The time-resolved fluorescence spectra at 77 K and 297 K indicate nearly similar decay profiles and the absence of dramatic spectral evolution in the overall spectral range ( Figures S20a and S20b), suggesting the lack of conformational heterogeneity. In contrast, the initial TA spectral features in frozen solution show different ESA structures compared to those in solution at room temperature ( Figures  S20c and S20d). The ESA band around 600 nm at 77 K decays with the time constant of 2 ps, while the biexponential rise of the ESA region at 297 K (4 and 16 ps) was observed. The initial state relaxes to the bright state with a time constant of 2 ps in the frozen solution, while the excimer state at the temperature above the melting point was formed by the ultrafast structural rearrangement.

RAS-2SF calculation
We performed quantum chemical simulations to characterize electronic states associated with the excimer. The ground state (S0) geometry obtained by DFT (ωB97X-D/6-31G(d)) was provided in the previous paper. The first excited singlet state (S1) geometry was optimized with time-dependent DFT with the same functionals and basis sets used for the ground state one. We would like to provide more detailed explanations on the selection of ωB97X-D for this work. It is well known that the charge transfer transition energy is underestimated by TD-DFT, which is due to the wrong asymptotic behavior of Coulomb potential. While generalized gradient approximation functionals and even hybrid functionals suffer from this problem, range-separated hybrid functionals can improve the description of asymptotic behavior of Coulomb potential by separating short-and long-range exchange interactions. The contribution of Hartree-Fock exchange increases with the interelectronic distance, and becomes 100 % at infinity. This approach recovers the correct asymptotic behavior of Coulomb potential, and can provide significant improvements for charge transfer energies. In this work, we employed one of range-separated hybrid functional, ωB97X-D, to optimize S0 and S1 structures. Also, ωB97X-D functional proves itself to be capable of providing quantitative understanding in the electronic structures of excited states of PBI molecules. 13,14 Therefore, we can expect that the S0 and S1 geometries of Bis-PBI optimized by ωB97X-D/6-31G(d) are quite reasonable, and can provide reliable descriptions of electronic structures.
Restricted active-space configuration interaction method with double spin-flip (RAS-2SF) calculations were performed to characterize the excimer-relevant electronic states in terms of diabatic states such as a ground state (GS), a local excitation (LE), a multiexciton state (ME) and charge resonance (CR). 15 We used the quintet reference state with four electrons in four active orbitals to include ME diabat which can play a role in stacked PBI structures. Even though RAS-2SF can achieve the balanced description of singly and doubly excited states, the lack of electron dynamic correlation results in overestimation of transition energies. Electron dynamic correlation can be partly recovered by the comparison with DFT energies. 13 TDDFT, constrained DFT, and unrestricted high-spin DFT computations were used to obtain local exciton, charge-resonance, and quintet state energies, respectively. The energies of adiabatic states from RAS-2SF with dominant local exciton, charge-resonance, and TT multiexciton diabatic character were adjusted to reproduce corresponding DFT energies. For DFT energies, we used ωB97X-D/6-31G(d). Table S3 and Table S4 list the corrected RAS-2SF energies at the ground state (S0) and the first singlet excited state (S1) geometries, respectively.
All quantum chemical simulations were performed with Q-Chem 5.1. 16